On MIMO SATCOM capacity analysis: Utilising polarization and spatial multiplexing

In this paper we present a MIMO model for a line-of-sight (LOS) satellite communication (SATCOM) channel, utilising orthogonal circular polarizations and spatial diversity from multiple satellites in geostationary orbit. To achieve spatial orthogonality in a LOS SATCOM channel, antenna separation of the order of hundreds of kilometres is required, either on the ground or in space. In this paper we consider small spacing between the ground terminals, and thus large separation between antennas in space by means of multiple satellites. Orbital slots for multiple satellites within two degrees are generally not viable due to adjacent satellite interference (ASI) issues from disadvantaged small user terminals. In this work we show how to benefit with MIMO spatial multiplexing and multiple satellites with closer orbital spaces. In addition to spatial multiplexing, orthogonal polarizations can double the overall bandwidth. In a SISO context, depolarization and cross talk between orthogonal polarizations are often expressed in terms of cross-polar discrimination (XPD) and cross-polar isolation (XPI) respectively. In a MIMO context, we show that polarization orthogonality is of more significance than antenna XPD or XPI. The capacity results show that significant improvement can be achieved with multiple satellites and polarization MIMO compared to conventional SISO SATCOM.